A composite sheet comprises a support and transparent solid fine particles attached thereon. The present specification hereinafter sometimes refers to the transparent solid fine particles as beads.
Japanese Patent Provisional Publication Nos. 6(1994)-347621, 6(1994)-347622, 6(1994)-347623, 6(1994)-347624 and 6(1994)-347625 disclose a composite sheet having the beads, which can be used as a retroreflective sheet. Glass beads are generally used in the retroreflective sheet.
The retroreflective sheet returns light to a light source. The sheet has been used on a surface of a guide-post, which preferably reflects light toward a light source, namely a headlight of a car. The glass beads function as fine spherical lenses in the retroreflective sheet.
The optical sheet having the beads can also be used as a light diffusing sheet. The sheet can be attached on a liquid crystal display (LCD) to enlarge the angle of field of the LCD, which usually has a narrow angle. The beads preferably have a diameter smaller than the matrix of the liquid crystal. The light diffusing sheet comprises a transparent film having a surface filled with such small beads. The light diffusing sheet can also function as a protective film of a polarizing plate. Accordingly, the number of the layers in the LCD can be reduced by using the sheet. The transmittance of the LCD increases with decreasing the number of the layers. The transmittance is the most important factor of the LCD. If one layer is deleted from the LCD, the transmittance increases by about 8%.
The light diffusing sheet can also be used as a screen for a projected image. An image projected from a liquid crystal projector has a problem of moire, which is caused by stripes of a color filter in the liquid crystal projector. The composite sheet having the beads can solve the problem by diffusing light while keeping the surface transmittance of the screen.
According to the prior art, a layer comprising a binder should be provided on a sheet to fix the beads.
Japanese Patent Provisional Publication Nos. 6(1994)-347621, 6(1994)-347622, 6(1994)-347623, 6(1994)-347624 and 6(1994)-347625 disclose a process for the preparation of the optical composite sheet, which comprises the steps of forming the layer comprising the binder on a support, arranging beads on the layer, and pressing them, for example between nip rollers to embed the beads in the layer.